The present invention relates to a method and apparatus that provides for wireless calls in private network environments and in public network environments. More particularly, this invention relates to inter-cell handover in wireless networks.
Conventional Cellular Systems
Present day cellular mobile telephone systems provide for a large and increasing demand for mobile services. Cellular systems “reuse” frequency within a group of cells to provide wireless two-way radio frequency (RF) communication to large numbers of users. Each cell covers a small geographic area and collectively a group of adjacent cells covers a larger geographic region. Each cell has a fraction of the total amount of RF spectrum available to support cellular users. Cells are of different sizes (for example, macro-cell or micro-cell) and are generally fixed in capacity. The actual shapes and sizes of cells are complex functions of the terrain, the man-made environment, the quality of communication and the user capacity required. Cells are connected to each other via land lines or microwave links and to the public-switched telephone network (PSTN) through telephone switches that are adapted for mobile communication. The switches provide for the handover of users from cell to cell and thus typically from frequency to frequency as mobile users move between cells.
In conventional cellular systems, each cell has a base station with RF transmitters and RF receivers co-sited for transmitting and receiving communications to and from cellular users in the cell. The base station employs forward RF frequency bands (carriers) to transmit forward channel communications to users and employs reverse RF carriers to receive reverse channel communications from users in the cell.
The forward and reverse channel communications use separate frequency bands so that simultaneous transmissions in both directions are possible. This operation is referred to as frequency division duplex (FDD) signaling. In time division duplex (TDD) signaling, the forward and reverse channels take turns using the same frequency band.
The base station in addition to providing RF connectivity to users also provides connectivity to a Mobile Services Switching Center (MSC). In a typical cellular system, one or more MSCs will be used over the covered region. Each MSC can service a number of base stations and associated cells in the cellular system and supports switching operations for routing calls between other systems (such as the PSTN) and the cellular system or for routing calls within the cellular system.
Base stations are typically controlled from the MSC by means of a Base Station Controller (BSC). The BSC assigns RF carriers to support calls, coordinates the handover of mobile users between base stations, and monitors and reports on the status of base stations. The number of base stations controlled by a single MSC depends upon the traffic at each base station, the cost of interconnection between the MSC and the base stations, the topology of the service area and other similar factors.
A handover between base stations occurs, for example, when a mobile user travels from a first cell to an adjacent second cell. Handovers also occur to relieve the load on a base station that has exhausted its traffic-carrying capacity or where poor quality communication is occurring. The handover is a communication transfer for a particular user from the base station for the first cell to the base station for the second cell. During the handover in conventional cellular systems, there may be a transfer period of time during which the forward and reverse communications to the mobile user are severed with the base station for the first cell and are not established with the second cell.
In time division multiple access (TDMA) systems, multiple channels are defined using the same carrier. The separate channels each transmit discontinuously in bursts which are timed so as not to interfere with the other channels on that carrier. Typically, TDMA implementations also employ FDMA techniques. Carriers are reused from cell to cell in an FDMA scheme, and on each carrier, several channels are defined using TDMA methods. The Global System for Mobile Communications (GSM), PCS 1900, IS-136, and PDC standards are examples of TDMA methods in current use.
The present specification uses a GSM system for purposes of explanation but the present invention applies to any wireless system protocol.
GSM Cellular Systems
The GSM system architecture is described, for example, in detail by M. Mouly and M.-B. Pautet, The GSM System for Mobile Communications, 1992 and Mouly and M.-B. Pautet, GSM Protocol Architecture: Radio Sub-system Signaling, IEEE 41st Vehicular Technology Conference, 1991. The following sections highlight some unique aspects of GSM systems.
The GSM system provides many advanced services, including: ISDN compatible supplementary services; Global roaming among GSM networks and other network types; advanced packet data services. The term ISDN includes narrow band (N-ISDN) and broad band (B-ISDN) where B-ISDN encompasses ATM and Frame Relay.
GSM System Architecture
There is a very comprehensive set of GSM Specifications which define the three major components of any GSM network, namely, the Mobile Station (MS), Base Station Sub-System (BSS) and the Network Sub-System (NSS).
Base Station Subsystem (BSS)
The Base Station Subsystem (BSS) is subdivided into two main entities, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). The BTS includes the radio transceivers that define the radio cell and supports the radio (Um) interface link to the mobile station. The BTS further supports the defined channel coding, encryption and speech coding functions. The BTS interfaces to the BSC via the Abis-interface.
The Base Station Controller (BSC) manages the radio resources of multiple BTSs. The BSC controls all of the functions related to the radio network, including the allocation and release of radio resources and control of radio interface handovers. The BSC interfaces to the MSC via the A-interface.
Network Subsystem
The network subsystem (NSS) comprises four components these are, the Mobile Services Switching Center (MSC), Home Location Register (HLR), Visitor Location Register (VLR), Authentication Center (AUC) and the Equipment Identity Register (EIR).
The main part of the network subsystem (NSS) is provided by the Mobile Services Switching Center (MSC). The MSC provides the ability to track the mobile user, switch calls to/from the user to the PSTN/ISDN/PLMN (public land mobile network) and maintain contact with the mobile user via radio handovers.
The GSM PLMN (GSM public land mobile network) contains two database functions related to the mobility of the user these are the Home Location Register (HLR) and the Visited Location Register (VLR).
The Home Location Register (HLR) is used by the operator to maintain data on all their subscribers. The subscriber data includes both fixed data, such as International Mobile Subscriber Identity (IMSI), subscriber MSISDN number and selected supplementary services, and dynamic data such as whereabouts of the mobile user and current settings for any supplementary services. Whenever a mobile users roams into a new Visitor Location Register (VLR) area the HLR downloads the subscriber data to the VLR The HLR is also required to deal with mobile terminating calls by routing the call to the correct VLR for further processing.
The Visitor Location Register (VLR) is used as a local cache to store the subscriber data near the current location of the mobile user. Once the VLR has downloaded the subscriber data the mobile user can begin to use the services provided by the network. The process of downloading the user data and establishing a presence in a particular location is called roaming.
In order to prevent fraudulent use of the network, the GSM PLMN also contains two further databases the Authentication Center (AUC) and the Equipment Identity Register (EIR).
The Authentication Center (AUC) is a maintained in a secure environment since it contains the network authentication algorithms. The network uses this database to obtain data that is used to “challenge” the mobile user when they make an access to the network. The AUC uses two algorithms known as A3 and A8, which are also stored in the Subscriber Identity Module (SIM) of the user. The A3 algorithm is used to authenticate the user by the familiar challenge response mechanism. The A8 algorithm is used to generate the required key for the radio interface encryption algorithm know as A5. Generally the A3 and A8 algorithms are developed by the network operators.
The Equipment Identity Register (EIR) contains a list of all the valid International Mobile Equipment Identity (IMEI) values. Using the unique IMEI value associated with all mobiles it is possible to restrict the use of specific mobiles. This database is used to prevent the use of stolen or misbehaving mobile stations.
Operations and Maintenance
Associated with the BSS and NSS equipment are Operations and Maintenance Centers, OMC-R and OMC-S, respectively. The OMC-R and OMC-S provide the following standard operations and maintenance control functions for the GSM BSS and NSS:
Configuration Management
Fault management
Test Management
Performance Management
Security Management
Account Management
Other functions may be available depending upon the BSS or NSS manufacturer. In addition, the OMC-S may also be required to perform database or user management functions on the HLR and VLR.
Basic Services Provided by GSM
GSM was designed to be a feature rich mobile services with most services based upon ISDN principles. In basic terms the GSM provides a basic set of services which can be enhanced by the use of supplementary services. The Basic Services include:                Speech, the most basic service        Short Message, a two way messaging service        Group 3 FAX, this services allows connection to Group 3 FAX machines        Cell Broadcast, this service allows messages to be broadcast to the mobile stations.        
The Bearer Services include:                Asynchronous Data 300-14400 bps, allows access to normal V-Series Modems        Synchronous Data 300-14400 bps, allows access to CSPDNs        General Packet Radio Services 9600-170000 bps, a packet services allowing seamless access to the internet.        
The following GSM supplementary services are currently available:                Call Forwarding        Call Barring Services        Call Transfer        Conference Calling        Call Wait and Call Hold        Calling Line Identification        Call Completion to a Busy Subscriber        
In private wireless networks, Mobile station access the network through the private base stations (P-BTS) using standard GSM protocols in the same way as they access the public wireless networks. But these private base stations are directly connected to a private LAN and employ Voice over IP technologies such as H.323 to route calls among Mobile station and between Mobile station and other PSTN and PBX terminals. In such private networks, typical inter-cell handover methods in a traditional public GSM network cannot be employed for handovers among private cells. Also, handovers between private cells and public cells requires new techniques for communication and handling between the MAP signaling based public cellular networks and voice over IP (VoIP) signaling based private cellular networks.
In accordance with the above background, it is the object of the present invention to provide wireless systems having the capability to have handover among cells in public and private networks.